The user evolution of wireless technologies has dramatically changed the way the user communicate and interact with their environment. The next generation of radio communications system, i.e. the fifth generation (5G) takes it even one step further by connecting not only individuals but also all sorts of devices and machines. One main objective of a 5G radio concept is to support highly reliable ultra-low delay machine-type communication (MTC), i.e. Critical-MTC. The Critical-MTC and wireless industrial automation concepts need to address the design trade-offs mainly between end-to-end latency and transmission reliability, and provide solutions for wireless networks suiting to different industrial application use cases. In almost all the Critical-MTC deployments, two distinct traffic patterns need to coexist, which include realtime and non-realtime traffic. The realtime traffic includes e.g. industrial control messages, alarms, alert messages, etc. while the non-realtime traffic mainly comprises software updates, transmission of log dumps etc. The wireless industrial automation system is needed to carry out radio resource management and govern channel access so as to ensure that realtime traffic requirements are fulfilled. Realtime traffic has the peculiarity that the message size is small, while non-realtime traffic typically consists of large data size. Realtime traffic could be periodic as well as sporadic. In wireless industrial automation systems such as Critical-MTC systems, the traffic patterns from different nodes are typically different and vary over time depending upon the application requirements and network dynamics. Traffic patterns on the same node can also be very different.
In many wireless industrial automation scenarios, schedule-based channel access schemes are currently used according to which channel resources are pre-allocated to all nodes (or communication links) in the network. In general, such pre-scheduled systems are overprovisioned and hence lead to resource (i.e., frequency and/or time) wastage when there is less traffic load. This inefficiency of resource utilization becomes increasingly alarming especially when the traffic patterns are unpredictable and varying. While most of the alarms and alert messages with realtime requirements cannot be predicted beforehand and are sporadic in nature, overprovisioning of resources results in precious bandwidth wastage in schedule-based channel access schemes. Furthermore, the need for pre-allocation or resources in schedule-based systems according to the network size, i.e. the number of nodes associated with a particular central node, tend to show a natural limitation in environments where the network size varies over time.
In contrast to schedule-based systems, contention-based systems in general suit better to dynamics in traffic patterns and network size variations. However, contention-based systems suffer from inability to assure traffic reliability guarantees in deterministic time scales especially with increasing system load or congestion in the network. In higher traffic load conditions and when multiple contenders attempt to access the medium, packet collisions may happen. Many contention-based systems like, for instance, Wi-Fi exercises a contention resolution mechanism through back-off algorithms to relieve contention situations and reduce packet collisions. Contention resolution schemes such as the one used by Wi-Fi systems increase latency for queued packet transmission and this becomes highly critical for realtime traffic.
Having the same access priority to the medium for both realtime and non-realtime traffic is naturally unfair for realtime traffic having strict timing deadlines. In Wi-Fi (particularly IEEE 802.11e), traffic access categories (AC) are used to prioritize certain types of packets over others. However, it is well known that despite using different service classes for channel access prioritization, Wi-Fi systems show acute disadvantage for VoIP traffic (with strict timing deadlines) when mixed traffic (VoIP, best effort, video, etc.) exist simultaneously in a network in the same shared medium.
Also, when large data is to be transmitted (i.e. MAC SDU (Medium Access Layer Service Data Unit) is larger than the maximum allowed frame size), Wi-Fi system splits it into smaller fragments and transmits them individually. Wi-Fi does not govern any prioritization for the transmission of fragments belonging to the same large data packet appearing over fragments of another large data packet generated at a later point of time. This leads to unfair channel access for traffic scheduled to be transmitted earlier especially when the traffic load is high.